Extruder assembly for extrusion of tetrafluoroethylene polymer tubes



Dec. 24, 1957 R. T. FIELDS 2,817,113 EXTRUDER ASSEMBLYFOR EXTRUSION 0FTETRAFLUROETHYLENE POLYMER TUBES Filed sept. 15. 1954 Fig. 1

REUBEN T. FIELDS ATTORNEY 1N VENTOR United States Patent O EXTRUDERASSEMBLY FOR EXTRUSION OF TETRAFLUOROETHYLENE POLYMER TUBES ReubenThomas Fields, Wilmington, Del., assigner to E. I. du Pont de Nemoursand Company, Wilmington, Del., a corporation of Delaware ApplicationSeptember 15, 1954, Serial No. 456,142

1 Claim. (Cl. 18-14) This invention relates to a device' for formingimproved tubing from tetralluoroethylene polymer and more particularlyrelates to an improved spider used in the tube extrusion oftetrafiuoroethylene polymer.

It is well known that polytetrauoroethylene is diliicult to fabricateinto various shapes by techniques applicable to most other plastics.Special techniques have` been developed and are known to those skilledin the art that make the fabrication of polytetrafluoroethylenepossible. These techniques are in general based on two properties of thepolymer. The low hardness of the polymer makes it possible to form thepolymer powder into a solid preform having the shape of the desiredarticle. This is followed by sintering the material above its meltingpoint to give a strong article. The extremely high melt viscosity of thepolymer prevents any major change of shape during the sinteringoperation. This process of manufacturing articles is described ingreater detail in U. S. Patent 2,400,099.

For the extrusion of polytetrauoroethylene into continuous shapes suchas rods, beading, tubes, and pipes, `a preferred method is to use alubricated polymer powder composition such as described in U. S. Patents2,593,582 and 2,685,707. The lubricant is added to the polymer to aid inthe deformation of the polymer particles, so as to obtain a solidpreform. Because of the high pressures needed, ram extruders arepreferred in the process of this invention. As in the continuousextrusion of other plastic tubing the extrusion of thepolytetratluoroethylene pipe preform requires a sleeve and a coretherein in order to form the polymer powder into a pipe. The core issupported at the inlet end of the sleeve by a spider which is soconstructed as to prevent the core from deecting from the center of thepipe and thus give a pipe with even thickness. The shape of this spiderin the general extrusion of plastic pipe is not very important withregard to the plastic being extruded, since most plastic materials areextruded above their melting temperature so that on passing through thespider the material is rewelded and no evidence of passing through thespider is found in the extruded product. In the extrusion oftetrafiuoroethylene polymer by the hereinabove mentioned method,however, the shape of the spider becomes of great importance, as thepolymer, extruded far below its melting point, will reweld only withditiculty to give a strong preform. As the polymer powder is alreadycompacted and under pressure when entering the' spider, the polymertends to form a seam when leaving the spider. T he same problem isencountered in the extrusion of tetrailuoroethylene polymer above itssintering temperature since the polymer will not weld with ease in themolten stage because of its high melt viscosity. For these reasons thespider used in the extrusion of polytetrafluoroethylene consists ofnumber of radial ns attached to a stem which in turn is attached to thecore. The tins are made as thin as the strength of the material willpermit without detracting from the primary objective of the spider,

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namely to keep the core suspended and centered. The extruded pipepreform will have a number of radial seams in it corresponding to thenumber of fins used in the spider. These seams must be strong enough towithstand the drying and sintering operations that follow the extrusionstep. Due to the thermal expansion of the polymer and the poor. strengthof the radial seam, cracks and splits occur occasionally in the pipe atthe seams in the aforesaid two processes. These cracks are often notdetectable during the manufacture of the pipe and may cause failureduring usage, which is expensive and hazardous.

It is therefore an objective of this invention to extrude stronger,unsintered pipe preforms from polytetrauoroethylene. Another object ofthis invention is to make awless sintered tubing frompolytetrafluoroethylene powder with uniform, high strength throughoutthe circumference of the tubing.

According to this invention, tubing of polytetrafluoroethylene havinggreater strength in the preformed unsintered stage as well as in thesintered stage is obtained if utes are introduced into the lower ends ofthe ns supporting the core in the sleeve of the extrusion die therebycreating non-radial seams in the extruded tubing.

The non-radial seams in the unsintered tubing will give rise to astronger bond in the sintering operation as the strains due to thethermal expansion of the polytetrailuoroethylene will not rupture theseams whereas in radial seams the forces of the thermal expansion of thepolymer will tend to pull the seam apart giving rise to splits andcracks.

This invention is further illustrated with reference to the accompanyingdrawings in which Figure l is the cross sectional view of the lowersection of a ram extruder showing the cylinder, the piston and theextrusion die with the core therein held in position by the spider.

Figures 2 and 3 show the lower edges of the ns of two types of spidersusable in this extrusion apparatus.

Figure 4 is the enlarged side view of one of the ns of the spidercontaining the flutes of this invention.

Figure 5 is the cross sectional view of a polytetrafluoroethylene pipepreform showing the resulting seam having increased strength.

As shown in Figure l of the drawings, the lubricated tetrauoroethylenepolymer powder 9 is forced by the piston 2 of the ram extruder 1, shownhere only in part, into the extrusion die 3. The polymer powder isforced by the pressure of the ram through the spider 5 and is compactedin the tapered upper end of the die 11 containing the spider 5. Theflutes 8 on the end of each side of each iin 6 result in seams in thetubing preform as illustrated in Figure 5. Upon passing through the ns 6the polymer is forced into the space between the sleeve of the die 13and the core 4 so that on leaving the die the polymer is shaped into apipe 10 consisting of highly compressed adhering particles oftetrauoroethylene polymer and lubricant. Although the seams are stillpresent in the preform as illustrated by the seams 12 in Figure 5 theyare no longer radial as would be obtained by using uniluted fins butgreatly increased in length and nonradial, thus imparting increasedstrength to the seam. The preformed pipe obtained from this extrusion isthen heated to remove the lubricant and sintered in separate operationsnot shown in the drawings since they are of prior art nature and knownto those skilled in the art.

In the sintered pipe obtained from the extrusion process of thisinvention the seams are no longer readily discernible. Tensile strengthmeasurements of samples of sintered pipe are the same in both verticaland horizontal directions indicating the awlessness of the pipe.

In Figures 2 and 3 are shown cross sectional views of the lower edges oftwo types of spiders used in the process of extrudingpolytetrafluoroethylene pipe, clearly indicating the edge effect of theflutes 8 on the fins 6 which are attached to the stem of the spider 7which in turn is attached to the core. The utes may be triangular innature as shown in Figure 2 or from a corrugated edge as shown in Figure3 without departing from the spirit of this invention of obtaining anon-radial seam in the preformed pipe.

In Figure 4 the enlarged side view of the tins containing triangularflutes is shown. The fin 6 is equipped with a tapered surface 14 at theouter edge so as to fit in the seat 11 of the extrusion die and thuskeep the core concentric with the sleeve as illustrated in Figure l. Theutes 8 are cut into the fin in such a manner as to create a triangularragged edge effect at the lower end of the n 6 as illustrated in Figure2. Since it is the object of this invention to create a non-radial seamthe length of the utes andthe number of the flutes may be varied withoutdeparting from the scope of this invention. It is preferred to use manyilutes since then the angles formed by the edge will be steep withregard to the radial direction, thus increasing the strength of theseam. The depth of the flute should increase gradually, leading up tothe hereinabove also for extrusions above the sintering tern- Catperature of the polymer. Furthermore, the present iuvention is notlimited to size or thickness of the polytetrafluoroethylene pipes ortubing produced. Having thus described the present invention it will beunderstood that details may be changed without departing from the `scopeof this invention.

l claim:

An extruder assembly for extruding polytetrailuoroethylene whichcomprises a source of polytetrafluoroethylene powder under pressure, adie and core defining an annular space, said die having a feed Zoneadjacent to said source and a forming zone removed from said source bysaid feed zone, lsaid die having a gradually decreasing diameter in thedirection of polymer ow in said feed zone and a constant diameter in thesaid forming zone, said core having also a constant diameter throughoutthat part of the forming zone which is adjacent to the said extrusionorifice, said core being rigidly held in place by a spider in said feedzone, said spider comprising radial fins on a central stem, said finshaving peripheral edges tapered to fit the die in the said feed zone,each of said fins having a continuous single serrated edge facing in thedirection of said orifice formed by triangular utes of graduallyincreasing width and depth in the direction of polytetrafluoroethyleneflow, said serrated edge being further characterized in that theserration is in a plane perpendicular to said central stern.

References Cited in the file of this patent UNITED STATES PATENTS1,679,545 Roth Aug. 7, 1928 2,363,261 Ritter Nov. 21, 1944 2,392,190Ritter Jan. l, 1946

